Field of Invention
This invention relates to an inertial sensor based surgical navigation system for knee replacement surgery that maps various anatomical geometry, tracks the position and orientation of the lower extremity in real time, calculates optimal cutting planes and implant positions and orientations, assists the surgeon with cut planning, and provides navigation for surgical instruments and implants using a graphical user interface.
Prior Art
Modern knee replacement surgery was revolutionized in the late 1960s and the early 1970s by a series of improvements in prosthesis design. Particularly, the total condylar knee developed by Insall and others at the Hospital for Special Surgery set a standard for knee implants for many years to come. Unfortunately while knee replacement designs continued to improve in the intervening years, the actual procedure and surgical instrumentation for knee replacement surgery has remained for the most part quite similar to the original methods and tools used by Insall in the early 1970s.
In the conventional method of knee replacement surgery, still used by about 90% of practicing orthopaedic surgeons, the alignment of surgical cuts is implemented using complex mechanical instruments that are at times highly invasive. For example, the most common method of referencing the mechanical axis of the knee is to use an instrument known as the intramedullary rod. The intramedullary rod is specifically designed to align the femoral cut guides to the mechanical axis of the knee. Using the intramedullary rod involves drilling a pilot hole into the femur, reaming out the intramedullary canal, and placing a rod inside of the canal. The use of the rod in this manner drills out excess bone, damages the bone marrow of the femur, and has been shown to cause greater blood loss during surgery than using non-conventional instrumentation. Furthermore, the rod is not always accurate. The intramedullary rod aligns itself to the anatomic axis of the femur that runs along the intramedullary canal. The anatomic axis is known to be between 5 and 7 degrees offset from the mechanical axis of the knee. The exact amount of deviation for a particular patient is guesswork, and some patients may fall outside of the standard deviation for various reasons making their particular mechanical axis very difficult to ascertain using the rod. The intramedullary rod can also occasionally fracture inside of a patient's femur causing major complications during surgery. Despite these problems, the intramedullary rod is a cornerstone of conventional surgical instrumentation used for knee replacement surgery.
Similarly, the extramedullary guide that is most commonly used to align the tibial cut guide in conventional surgery also has problems. The extramedullary guide is a large mechanical jig that runs parallel to the length of the tibia. Its purpose is to reference the anatomic axis of the tibia for the proximal tibial cut. Unlike the intramedullary rod however, the extramedullary guide remains on the outside of the bone. Although less invasive than its intramedullary counterpart, most designs of the guide suffer from a lack of adequate fixation. In other words, there is little actually holding the large mechanical jig in place other than the hands of one member of the surgical team. This is a problem because even a few millimeters of misalignment have been shown to have negative effects upon implant longevity. Using this type of free-floating guide and methodology, it is difficult to produce repeatable and accurate results when aligning the tibial cut guide for the proximal tibial cut.
In both situations, the mechanical jigs usually only allow for the alignment of the cut guides in the coronal plane, or from a front view of the knee. The alignment of the cut guides from the sagittal perspective, or from a side view, often has almost no means of verification other than the surgeon's eye.
Another problem with the conventional instrumentation and methodology is that most of the cut planning is done prior to the surgery, using standing x-rays. The problem with a standing x-ray is that if the stance of the patient is slightly rotated during the x-ray, meaning that if the patient's foot is not pointed straight ahead, the degree of varus/valgus deformity can be misjudged.
In short, the problems with conventional instrumentation is that the tools used for cut-planning and for aligning surgical cuts are only designed to work from the coronal perspective, and leave much room for human error producing results that are not always repeatable. Furthermore some of the instrumentation used, such as the intramedullary rod, is highly invasive.
By the late 1990s and the early 2000s the first surgical navigation systems came into being. The premise behind a surgical navigation system is that a sensor or group of sensors can align surgical cuts in a precise and accurate manner, while being less invasive than the conventional mechanical instrumentation of the past.
The surgical navigation systems for the most part use a camera and a set of optical trackers to track the position and orientation of the bones and surgical instruments during surgery. In most optical navigation systems, infra-red beams are shot from a camera to a set of reflective trackers, which then reflect the beams back to the camera. The reflection of the beams provides the computer system with information about the exact location of the trackers. In other optical navigation systems, a light emitting diode is fixed directly to the optical tracker, which then actively shoots infra-red beams to the camera. There are also numerous other forms of emitter/detector schemes that have been attempted, such as using ultra-sound in place of infra-red beams. In practice these navigation systems are virtually identical.
The trackers are used by the computer system to provide motion tracking information regarding the position and orientation of the bones and surgical instruments. A specialized point registration tool is used in these optical navigation systems, to map anatomically significant points on the bones into computer memory. The position and orientation of these points are saved, and their movement is tracked by the computer system. The registered points are used to generate a virtual model of the femur and tibia, and to calculate specific geometry of interest such as the joint centers of the hip, ankle, and knee, and the mechanical axis of the knee. This data is in turn used to calculate the optimal cutting planes to which the cut guides should be aligned for surgery. A graphical user interface on the computer screen is then able to instruct the surgeon exactly how to place the cut guide on the patient's knee without the use of the complex mechanical jigs of conventional surgery. This process is known as navigation. The navigation process has the advantage of aligning cut guides accurately and in a repeatable fashion, without being as invasive as conventional instrumentation. The computer is also able to provide the surgeon enough numerical and visual feedback, such that the cut guides can be aligned in both the coronal and sagittal planes, which the conventional instrumentation is also not capable of matching. Although the new computer based surgical instrumentation has solved many of the problems with the conventional instrumentation of the past, it also created some completely new ones.
The cameras in these systems typically have very specific constraints as to the angle and distance at which they must be located in relation to the operating table. In the Aesculap system for example, the camera has to be at a roughly 45 degree angle to the table, and must lie generally about two meters away from the knee. If the camera is placed outside of these constraints, it will not pick up the optical trackers. Furthermore, if the knee is moved such that the trackers are at an acute angle to the camera, the camera will have difficulty recognizing them. Also, the field of the camera is occasionally not wide enough to adequately capture all of the optical trackers from the hip to the ankle. This occurs when the trackers are not positioned on the bones well at the beginning of surgery. Finally, blood or tissue can also sometimes cover the reflective material or LEDs on the trackers, obstructing the camera's line-of-sight.
The trackers are purposefully designed to be large so that the camera can track them. A typical size envelope for a tracker is about 6 inches by 4 inches by 4 inches. As a result, they can quite easily be knocked off the bones inadvertently by a surgeon or nurse's elbow. The sheer size of the trackers can also make the angle of approach for other surgical instruments into the knee awkward and difficult to manipulate around.
In actual practice, line-of-sight between the camera and the optical trackers is a serious issue. In a typical surgery, there are multiple people hovering around the operating table that can physically obstruct the line-of-sight to the optical trackers. To avoid this from happening, one entire side of the operating table is off limits as to where the surgical team can stand. This is a real limitation that cannot be understated. Manipulating instruments carefully into a person's knee requires a fair amount of room, and two or three people crowded on the other side of the operating table creates an additional element of difficulty in the surgery.
In short, the optical navigation systems can align cut guides with greater precision and accuracy than conventional instrumentation. The optical navigation instrumentation is also far less invasive and simpler to use, because a graphical user interface on a computer screen renders the complex mechanical instrumentation of conventional surgery obsolete. The downside however is that line-of-sight issues as well as the large size of the optical trackers make surgery with an optical navigation system awkward and difficult at times.
Finally, in the mid 2000s a few inventors tinkered with the idea of using inertial sensors in surgery. Inertial sensors are sensors that are capable of detecting their own motion without the use of any external references. One reason why it may have taken so long for inertial sensors to trickle into the operating room is because previously the major applications for inertial sensors involved high speed vehicles. For example, inertial sensors are used to measure the acceleration of automobiles in order to deploy airbags, and the acceleration and angular velocity of jet planes for navigation purposes. Also the inertial sensors are generally not designed to take direct position and orientation measurements. A number of mathematical algorithms must first be applied to the raw data from the inertial sensors to derive the position and orientation information.
Haid (US 2007/0287911) describes the use of inertial sensors in surgery, and states how a series of quaternion algorithms and kalman filter algorithms might be used to derive accurate position and orientation coordinates from an inertial sensor. The Haid system does not describe how inertial sensors might be used to align surgical instrumentation or implants.
Wasielewski (US 2004/0243148) mentions attaching inertial sensors to various surgical tools and implants in knee replacement surgery. Unfortunately however, the invention Wasielewski describes does not calculate the optimal cutting planes from anatomically significant points, and thus does not actually navigate the cut guides using a graphical user interface. In fact, Wasielewski's system explicitly depicts the use of the intramedullary rod to reference the mechanical axis of the knee. Basically, Wasielewski describes a system which is comprised of a set of conventional instrumentation with inertial sensors attached. The drawback of Wasielewski's system is that it has all of the same flaws that the conventional instrumentation and methodology has. Furthermore, the system Wasielewski describes, mounts inertial sensors directly to anatomically significant points on the bones which is even more invasive than the conventional procedure.
Finally, Proulx (US 2009/0247863) also describes the use of inertial sensors to track the position and orientation of surgical instruments and implants. Proulx's invention, like Wasielewski's, does not calculate specific geometry like the mechanical axis of the knee from registered anatomical points, nor does it calculate the optimal cutting planes or component positions for surgical instrumentation. In other words, it also does not navigate cut guides or implants using a graphical user interface.